1. Field of the Invention
The present invention relates to a solid-state image pickup apparatus capable of reading out images signals while thinning them down in the horizontal direction, and a signal reading method therefor. More particularly, the present invention relates to a solid-state image pickup apparatus capable of processing image signals representative of, e.g., a scene picked up and outputting the processed image signals while using the image signals for a control purpose. The present invention is desirably applicable to, e.g., a digital still camera, image input apparatus or similar imaging apparatus of the type automatically adjusting a focus and an exposure with image signals representative of a scene picked up.
2. Description of the Background Art
An imaging apparatus of the type described includes an image pickup section having CCDs (Charge Coupled Devices) or similar photosensitive devices each forming a single pixel or cell. Today, the number of pixels required of the image pickup section is increasing to meet the increasing demand for higher image quality. This, however, increases a period of time necessary for image signals to be read out of the image pickup section.
Japanese patent laid-open publication No. 136244/1998, for example, teaches an electronic image pickup apparatus capable of reducing the above period of time for driving an image pickup section. The image pickup apparatus taught in this document implements the display of a movie with a drive frequency of less than 20 MHz despite that it uses an image sensor having about 1,000,000 CCDs. Specifically, the apparatus uses color filters arranged in a Bayer pattern in combination with the CCD image sensor. In a high speed mode, the apparatus drives the image pickup section in such a manner as to output one line of image signals every three lines, i.e., to thin down the lines in the vertical direction. This is successful to read image signals at a high speed with a low drive frequency. Particularly, when automatic focus (AF) adjustment, automatic white balance (AWB) adjustment and automatic exposure (AE) adjustment are effected during movie display, control data is required frame by frame. In this case, the apparatus selects a mode for adding three consecutive lines of signals and outputting the resulting sum. In this manner, to reduce the signal reading time, the apparatus thins down the lines in the vertical direction. For the AF, AWB and AE adjustment, the apparatus adds up three consecutive lines of signal charges in order to produce a control signal, i.e., reads all of the three primary colors R (red), G (green) and B (blue) separated by the color filters and then combines them.
As for the generation of control data for AE and AF, a luminance signal plays an important role, as well known in the art. In addition, in a solid-state image pickup apparatus, only the color G contributes to the generation of a luminance signal, as also generally accepted. For photometry, therefore, only the color G may be read out in order to promote effective signal reading. However, it is difficult with the Bayer arrangement to read only the color G because the colors R, G and B exist together in an array.